The present invention relates to processes for protecting a radar against jamming making it possible to obtain a correct detection, despite intentional spurious transmissions for the purpose of jamming this detection. It also relates to radars, which have been especially adapted to put this process into effect.
It is known to jam a radar by transmitting, preferably on the wavelength used by it, clutter signals which, superimposed on the echoes which it receives, disturb the operation of the receiver and/or analysis equipment for the signals detected by this receiver. The best known process consists of transmitting a high-powered, continuous signal on the frequency of the said radar and this completely saturates the radar receiver.
To escape from this jamming action, radars of the so-called frequency agile type have been developed and it is possible to vary the transmission frequency thereof by selecting this frequency, either manually or automatically, as a function of the environment from a number of preselected frequencies. This device also makes it possible to prevent disturbing another radar forming part of the surveillance system in question.
Improvements to radars and jammers have taken place simultaneously, each alternatively being at an advantage compared with the other. At present, countermeasure systems comprising analyzers for determining the transmission frequency of the radar to be jammed and for controlling the jammer on the basis thereof, are able to acquire the frequency of the radar on the basis of a very small number of pulses and specifically two or three pulses. To overcome this obstacle it is necessary to be able to change the radar transmission frequency from one pulse to the next. This is difficult to obtain, although fundamentally achievable. However, under these conditions, it becomes impossible to carry out Doppler processing on the pulses received, because the frequency must remain fixed during the processing of a plot, which is obtained by the processing of a pulse train.